1. Field of the Invention
The invention relates to thin film dielectric coated optics for use in a resonant cavity of a laser.
2. Description of the Related Art
The conventional laser resonating cavity includes a series to two reflecting optical elements placed at either end of a laser tube. In general, the optical elements are coupled to the laser tube in an external manner; that is, the mirrors are mounted to the ends of the tube, with a glass frit, or solder glass joining the reflective portion of the element directly to the end of the laser tube. During the glass frit process, temperatures can reach up to 485.degree. C. U.S. Pat. No. 4,893,314, inventors Shull, et al., assigned to Spectra-Physics, Inc., shows a mounting alternative wherein the laser optics are placed on an internal mount. A mirror seat is inserted into the interior of the laser bore and the optical element is mounted on the mirror seat with the reflective portion of the element on the opposite side of the mounting area.
Optical elements for a laser resonator may be manufactured by coating a substrate, such as glass, with a series of dielectric films to develop the desired reflectance/transmittance of the mirror, depending on whether the mirror is to be used as a high reflectance mirror or as an output coupler. As is well known, such coatings are generally comprised of a plurality of layers of dielectric material, the layers in a reflective stack alternating between materials with high and low indices of refraction with each layer being typically about .lambda./4 in optical thickness, thereby defining a reflective surface. The number, index of refraction, and optical thickness of the layers is determined by the desired reflectivity or transmittance of the optical element.
In ion lasers where a "white light" output is desired, operation over a broad range of wavelengths--the red, green and blue visible regions of the spectrum--is required. Three coatings, fabricated using thin film techniques, require a large number of layers for proper operation over broad wavelength ranges due to the particular materials which must be used to make the layers. Normally, the number of layers is limited to 25-40 layers because the materials which may be used to fabricate the coatings must withstand a high temperature frit process to couple the optic element to the laser tube. In general, the thicker the stack of films, the greater the potential for craze.
It is well known that it is possible to select particular transmission bandwidths in, for example, bandpass filters, by varying the structure of the layers deposited on the optical substrate. One such technique involves using absentee layers comprised of contiguous layers of low index materials, having an optical thickness which is approximately twice that of the other layers in the stack. The use of absentee layers provides higher absorption in particular spectral regions. In general, such absentee layers provide for narrow, triangular transmission and absorption spikes on the spectral curve (spikes occur at the same wavelength), and are widely utilized in the manufacture of narrow bandpass filters as it is possible to block all but one transmission peak using absorbing filter or varying the structure. In a quarter wave (.lambda./4) stack, the absentee layer has a one-half wavelength optical thickness (.lambda./2), placed at the center of the quarterwave stack, which has no effect on the reflectance of the particular wavelength under consideration (assuming zero absorption loss). In such narrow bandpass filters, the objective is to render the transmission spike extremely narrow and with an extremely high transmittance. However, in such designs the potential for unwanted absorption spikes in the filter bandwidth must be continually avoided.
Use of such absentee layers thus provides a technique for the selection of certain regions of operation of the optical element over which the characteristics of the element can be closely controlled.
Other coating designs have incorporated multiple wavelength quarterwave stacks to create white light laser resonators. In a white light laser, the optics must have high transmission in three spectral regions, at 786-622 nm (red), 577-492 nm (green), and 492-455 nm (blue). In such designs, the optical thickness of each layer may vary to create two or more "stop bands," or regions of the wavelength spectrum in which the optic is substantially reflective, separated by regions where the optic is substantially transmissive.
Other problems faced by optical coatings result from intra-cavity effects. Exposure to ultraviolet radiation and vacuum pressures causes deterioration of the coating material. Coating materials which can withstand such effects, such as hafnium dioxide (HfO.sub.2), have relatively low indices of refraction compared to commonly used metal oxide films such as TiO.sub.2 and Ta.sub.2 O.sub.5 and a large number of layers are required to fabricate broad bandwidth coatings.
A further problem faced by all coatings is spectral shift during the final sealing of the laser tube. Generally, all dielectric coatings are subject to shift as water molecules are present in the somewhat porous dielectric coatings. A low temperature sealing bake of the assembled tube is performed to outgas foreign materials which may have entered the tube during manufacture. When exposed to the vacuum and final bake, the water molecules exit the coating. Because water has a higher density than air, the operating range of the coating will shift approximately 1-2% shorter. Thus, coatings are generally designed to be manufactured to operate in the spectral region at approximately 1-2% longer wavelengths than desired so that after baking, the coating operates in the desired region.
It is thus desirable to provide a novel optical coating having reflectivity over a broad bandwidth which may be selectively tuned to the particular bandwidth for operation of the laser.